1. Field of the Invention
The present application relates to an image forming apparatus, an image forming method and a process cartridge for use in facsimile machines, photocopiers and printers.
2. Discussion of the Background
Unexamined published Japanese patent application No. (hereinafter referred to as JOP) 2001-054571 describes an image forming apparatus developed to restrain the occurrence of image blur to provide a long-life image bearing member. Image blur is caused by accumulation of nitrogen oxides ascribable to a lubricant supplied to the surface of an image bearing member. The image forming apparatus has a structure in which a lubricant supplying device supplies a lubricant to a photoreceptor functioning as an image bearing member to make the friction coefficient of the surface of the photoreceptor low. A process of reducing the surface friction coefficient of the photoreceptor and a process of increasing the surface friction coefficient of the photoreceptor are described. In the process of increasing the surface friction coefficient of the photoreceptor, nitrogen oxides produced during charging are removed. In the process of increasing the friction coefficient of the surface of the photoreceptor, for example, the contact pressure of the cleaning blade to the photoreceptor is greater than in the process of decreasing the surface friction coefficient.
In the technology described above, the surface friction coefficient of the photoreceptor is altered by changing, for example the contact pressure of the cleaning blade, to remove nitrogen oxides produced during charging. However, it was found recently that it is difficult to prevent deterioration of image quality such as image blur simply by removing the nitrogen oxides. In addition, in the technology described above, there is no mention that the lubricant protects a photoreceptor. Also there is no mention that the lubricant may be deteriorated by charging.
Further, JOP 2001-228668 describes an image forming apparatus in which a plurality of image bearing members are arranged along the transfer direction of a transfer material. Toner images formed on the surface of each image bearing member are transferred to the transfer material and the remaining toner attached to the surface of each image bearing member after the toner image is transferred to the transfer material is removed by a cleaning device. Further, a toner retrieving device is provided in the image forming apparatus to return the remaining toner removed by the cleaning device to the cleaning device and to prevent deterioration of image quality by paper dust contained in the remaining toner. That is, JOP 2001-228668 describes an image forming apparatus in which the toner retrieving devices are provided to the image bearing members except for the image bearing member disposed on the uppermost stream side from the transfer material. The toner retrieving devices are disposed between the cleaning devices to remove the remaining toner from the image bearing members and the developing devices. However, the technology described above does not refer to removal of the lubricant on the surface of the image bearing member.
In addition, JOP 2002-6689 describes an image forming apparatus in which a lubricant is supplied to the image bearing member on which toner images are formed to elongate the life of the image bearing member and to improve the image quality. Therein, the number of rotations n of the image bearing member is preset for the surface friction coefficient of the image. During image formation, when the number of the rotation of the image bearing member has reached a number n+A (A is an integer), the transfer current at the transfer portion is increased without performing image formation to increase the amount of the lubricant retrieved from the surface of the image bearing member. Thereby, the friction coefficient of the surface of the image bearing member is temporarily raised so that the product produced resulting from charging can be removed. After removing the product produced from the charging, the transfer current is returned to a level suitable for image formation and the image bearing member is rotated without forming images to apply the lubricant to the surface thereof up to a suitable amount. Thereby, the surface friction coefficient of the image bearing member is reduced. Thereafter, images are formed while applying the lubricant.
To raise the friction coefficient of the image bearing member, the technology described above describes a technology for removing nitrogen oxides together with the lubricant by changing the polarity of the transfer bias. It was recently found that it is difficult to prevent deterioration of image quality such as image blur simply by removing the nitrogen oxides. In addition, in the technology described above, there is no mention or concept that the lubricant protects a photoreceptor. Also there is no mention about deterioration of the lubricant by charging.
Further, JOP 2002-357983 describes a lubricant supplying device and an image forming device wherein the lubricant is used. In the device, the lubricant is sufficiently supplied without causing the increasing cost due to the increases of the mounting space of a member to supply the lubricant and the number of parts. The lubricant supply device is provided to supply the lubricant to the surface of a photoreceptor. The lubricant supply device has a blade-shaped member in which a lubricant is inclinationally dispersed in a rubber-like elastic member to make the lubricant present in a large quantity on one surface side than the other. In addition, the lubricant supply device supplies the lubricant to the surface of a photoreceptive drum by bringing the side of the blade-shaped member having the lubricant in a large amount into press-contact with the surface of the photoreceptor.
However, the technology does not refer to the method of supplying a lubricant to the photoreceptor in which the lubricant is contained in parts to dispense with the space for a solid or powder lubricant. In addition, there is no mention about removing the lubricant on the surface of the photoreceptor.
Applying a lubricant to a photoreceptor is also a means by which a protective layer may be added, such as of an image bearing member of an image forming apparatus to provide, for example, a long life of the image bearing member and quality images. The objects of the application of a lubricant are to prevent the occurrence of toner filming (fusion attachment), improve transfer efficiency by reducing the friction coefficient and prevent poor cleaning performance. JOPs 2002-244516, 2002-156877, 2002-55580, and 2002-244487 describe technologies related thereto.
JOP2002-229227 describes a technology to improve anti-abrasion property by applying a lubricant containing zinc stearate to a photoreceptor using a non-contact charging device to obtain a long-life charging member and photoreceptor. In the technology, organic particulates are dispersed in the photoreceptive layer of the photoreceptor. JOP H10-142897 describes an image forming apparatus having a blade form supplementary member. The blade form supplementary member is provided to even out the lubricant applied at the portion between the charging portion and the developing portion and to stem lubricants having a large particle diameter.
The widely-used known cleaning method for an image forming apparatus in typical electrophotography is a method in which a cleaning blade is used. There are a number of image forming apparatuses having only a blade as a cleaning device. In addition, in the case of a high speed electrophotographic machine, to avoid a state in which a large amount of toner is locally attached, an image forming apparatus is proposed in which a brush is provided on the upstream from the blade. However, such technologies have a drawback in that it is impossible to sufficiently remove the recently developed (polymerized) toner having a circularity of 0.96 to less than 1.00. In spite of this, there are short life image forming apparatuses which can remove such toner by giving some devise to toner and a blade. In addition, spherical (polymerized) toner has a high transfer ratio, meaning that the amount of the remaining toner is small. Therefore, there is proposed an image forming apparatus in which the developing device performs cleaning without a dedicated cleaning device.
In addition, a polarity control device provided on the upstream side from the cleaning device was used in an old-type image forming apparatus but few of them are now seen. One of the reasons is that such a polarity control device is no longer required due to improvements in cleaning technology and a desire for cost reduction. Among the cleaning devices having a brush on the upstream side from the blade, some cleaning devices also function as a polarity control device to which a voltage is applied but they are not popular. However, there are many image forming apparatuses having a polarity control device when the cleaning device mentioned above is not provided thereto.
As a charging device to charge the image bearing member of an image forming apparatus, the charging device using corona discharging used to be popular. However, this charging device using corona discharging has a drawback in that such a charging device produces ozone in a large amount. In addition, since a high power source is required to apply a voltage as high as 5 to 10 kV to perform corona discharging it is difficult to reduce cost of an image forming apparatus.
Therefore, contact type charging devices, in which a charging member contacts an image bearing member, have been adopted in many cases instead of a corona discharging device. This contact-type charging device can solve most of the drawbacks involved in the charging devices using the corona discharging mentioned above. On the other hand, the contact-type charging device invites problems such as abnormal images referred to as image deletion and increased abrasion of the image bearing member. In addition, when AC is used as an application voltage, noises occur, which also becomes a problem. In addition, the charging device rubs toner, paper dust, etc., with an image bearing member (photoreceptor), which accelerates contamination. Work-up in printing stemming from this contamination creates another printing problem. To solve these kinds of problems, JOP H10-312098 describes a technology in which the contamination due to toner and paper dust caused by a charging device mentioned above is prevented by controlling an applied voltage using a supplementary charging member and a charging member. In addition, by this technology, the occurrence of abnormal images referred to as positive-ghost in a cleaner-less system can be prevented.
As mentioned above, applying a lubricant to the image bearing member of an image forming apparatus is widely performed to improve transferability and/or cleanability. However, when an image bearing member is charged by a charging device having a charging member disposed in the vicinity of or contacting with the image bearing member, the lubricant on the image bearing member deteriorates due to the charging and the amount of the lubricant on the image bearing member decreases. As a result, it is difficult for the lubricant to carry out its function. Moreover, if the degraded lubricant is left on the image bearing member and not removed, the lubricant gradually accumulates. Thereby, the image quality deteriorates and abnormal images occur. There is a problem that typically used cleaning blades or cleaning brushes cannot sufficiently remove the degraded lubricant on the surface of the image bearing member by a charging device.